1. Field of the Invention
The present invention relates to a traction control system of a motor vehicle, and in particular to a traction control system for a motor vehicle with a catalytic converter in an exhaust path of an engine.
2. Description of Related Art
An excessive driving torque generates a slip in a driven wheel when the acceleration of a vehicle is necessary. When the slip occurs in the driven wheel, the acceleration of the vehicle is decreased. In order to prohibit such decrease of the acceleration, the conventional motor vehicle is provided with a traction control system.
The traction control system decreases the engine output and/or increases the braking force so as to reduce the slip amount in the driven wheel to a predetermined amount, when the excessive slip occurs in the driven wheel. The retard of ignition timing and the fuel cut has been carried out in order to decrease the engine output. That is, the control levels are provided based on the slip amount in the driven wheel, and thereafter the ignition timing is retarded and the fuel cut in the predetermined cylinders is carried out based on the table whose parameter is the control level.
On the other hand, the motor vehicle is provided with a catalytic converter which lowers emission levels of unburned hydrocarbons, carbon monoxide and the like. Where the traction control system decreases the engine output by the retard of ignition timing and the fuel cut when the excessive slip occurs in the driven wheel, the following problems occur.
When the ignition timing is retarded, the amount of unburned hydrocarbons in the exhaust gas relatively increases and the after burning occurs for a long time period which causes the temperature rise of the exhaust gas. Therefore, the oxidation in the catalytic converter becomes active and the temperature of the converter becomes high. As a result, the characteristics of the catalytic converter decreases and the durability of the converter decreases because of the heat deterioration of catalyzer. On the other hand, when the fuel cut is carried out, the hydrocarbons discharged from the cylinders without the fuel cut contact with the thick oxygen in the exhaust gas discharged from the cylinders with the fuel cut before reaching the catalytic converter and have possibility of reburning. As a result, the exhaust gas is heated again which causes the same problems explained above.
In order to resolve such problems explained above, Japanese Patent Laid-Open No. 5-1613 discloses the traction control system for the motor vehicle with the catalytic converter in the exhaust path. According to the traction control system disclosed, when the engine speed is greater than a predetermined speed, the engine output decreasing patterns are shifted from the usual pattern to another pattern by which the temperature of the exhaust gas is forced to be decreased. More specifically, since the intervals of the exhaust gas discharged from the respective cylinders reaching the catalytic converter becomes short at the high speed of the engine, the abnormal temperature rise of the exhaust gas can be avoided and therefore the above-mentioned problems are resolved.
However, there are still problems to be resolved in the conventional traction control system disclosed in Japanese Patent Publication above, as explained below.
There is a motor vehicle in which a high octane number gasoline is to be used for an engine. Among the motor vehicles in which the high octane gasoline is designated, there are vehicles whose engines can be operated safely even if low octane number or normal gasoline is actually used. This is because the control unit of the engine memory is a first ignition timing map for the high octane number gasoline and a second ignition timing map for the regular gasoline in which the ignition timing is provided in a retard direction. Further, the first ignition timing map is usually used, and the first ignition timing map is switched to the second ignition timing map when the knock sensor detects that the regular gasoline has been used.
In a vehicle having such first and second ignition timing maps, suppose that the vehicle is traveling with an engine condition of high load and low engine speed. If the regular gasoline is used, then the ignition timing is controlled by using the second ignition timing map for the regular gasoline in which the ignition timing is provided in a retard direction. If the traction control is started under this condition, the ignition timing is provided in a retard direction by the traction control because the engine speed does not become high. Therefore, the ignition timing is retarded in a large amount by both of the second ignition timing map for the regular gasoline and the traction control. Further, the unburned hydrocarbons in the exhaust gas increase because the fuel amount to be supplied increases under the high load of the engine, the temperature in the exhaust gas increases because the after burning continues for a very long period. As a result, according to both the increase of the hydrocarbons in the exhaust gas and the after burning for the very long period, the temperature of the catalytic converter becomes very high because the oxidation of hydrocarbons becomes very active. Finally, characteristics of the catalytic converter increases and heat deterioration in the catalyzer occurs.
Where the vehicle has an engine with a supercharger such as a turbo-charger or a mechanical supercharger, the same problems occurs when the engine is in the condition of high load and low speed.
That is, when the ignition timing is retarded by the traction control under the condition that the engine has been supercharged, the after burning is promoted and the temperature of the exhaust gas becomes of very high because the large amount of air forced into a combustion chamber of the engine and the oxidation of hydrocarbon in the catalytic converter under the thick oxygen. Therefore, according to both of the high temperature the exhaust gas and the oxidation of hydrocarbon under the thick oxygen, the oxidation of hydrocarbon is abnormally promoted and the temperature of the exhaust becomes extremely high. As a result, the characteristics of the catalytic converter decreases and the heat deterioration of catalyzer occurs even if the engine speed is not so high.